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Hydrogenation of Alkenes with Nabh4, CH3CO2H, Pd/C in the Presence of O- and N-Benzyl Functions

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Hydrogenation of Alkenes with Nabh4, CH3CO2H, Pd/C in the Presence of O- and N-Benzyl Functions International Journal of Organic Chemistry, 2016, 6, 1-11 Published Online March 2016 in SciRes. http://www.scirp.org/journal/ijoc http://dx.doi.org/10.4236/ijoc.2016.61001 Hydrogenation of Alkenes with NaBH4, CH3CO2H, Pd/C in the Presence of O- and N-Benzyl Functions Nuha Al Soom, Thies Thiemann* Department of Chemistry, United Arab Emirates University, Al Ain, Abu Dhabi, United Arab Emirates Received 13 December 2015; accepted 1 March 2016; published 4 March 2016 Copyright © 2016 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ Abstract NaBH4, CH3CO2H, Pd/C has been described as an effective reagent system to hydrogenate alkenes. Here, we show that the hydrogenation occurs chemoselectively, making it possible to hydrogenate alkenes under Pd/C catalysis with hydrogen created in situ without O- or N-debenzylation. Keywords Alkene Hydrogenation, Benzyl Ether, Benzyl Ester, N-Benzyl Group 1. Introduction One of the most common procedures for the hydrogenation of alkenes in a chemistry laboratory is the hy- drogenation over palladium catalysts such as over palladium on carbon (Pd/C). Because of the danger of work- ing with H2 in our laboratory, we looked for a reaction system that would generate H2 in situ. Recently, A. T. Russo et al. [1] [2] have published reaction conditions (NaBH4, CH3CO2H, Pd/C) that would achieve this. We could utilize this system, e.g., in the hydrogenation of 1 to 2 (Scheme 1), where 2 is a precursor to quinines 3 linked to a carrier. At the time, we exchanged the published solvent of the reaction, toluene, to the more easily removable benzene. One of the common protective groups for the alcohol (OH) function and for the carboxylic acid (CO2H) func- tion is the benzyl moiety in form of an O-benzyl ether (OCH2Ph) and O-benzyl ester (CO2CH2Ph) [3]. Often, both can be removed by hydrogenolysis when using a palladium on carbon (Pd/C) catalyst [4]. Also, an N-func- tion, such as in an amide, can be protected with a benzyl group, where the group is subsequently removed by Pd-catalysed hydrogenation. Under the conditions of the reductive debenzylation, double bonds can also be *Corresponding author. How to cite this paper: Soom, N.A. and Thiemann, T. (2016) Hydrogenation of Alkenes with NaBH4, CH3CO2H, Pd/C in the Presence of O- and N-Benzyl Functions. International Journal of Organic Chemistry, 6, 1-11. http://dx.doi.org/10.4236/ijoc.2016.61001 N. A. Soom, T. Thiemann NaBH OMe 4 OMe O CH3CO2H Pd/C benzene MeO CO2Me MeO CO2Me O CONHR 1 2 3 Scheme 1. Olefin hydrogenation with NaBH4, CH3CO2H, Pd/C. hydrogenated, of course. If the actual desired transformation, however, is to be the hydrogenation of a double bond in the substrate, then one risks losing the benzyl functions as protective groups in the molecule at the same time. Oftentimes, the hydrogenation reaction is not chemoselective, but coincides with the reduction of nitro groups, azide functions, dehalogenations and also with O- and N-debenzylations of O-benzyl ethers and esters, N-benzyl amines and amides. In recent times, more chemoselective catalysts have been developed, mainly based on platinum group metals. These catalysts include polymer-imprinted platinum [5], ZnX2-Pd/C and Pt/C systems [6] and platinum sulfides [7], and specifically prepared Pd-catalysts [8]. Also, the addition of amines [9] [10] or diphenyl sulfide [11] to Pd/C or Pt/C has been found to make the catalysts more chemoselective, where the hy- drogenation of alkenes is not accompanied by all of the side reactions mentioned above. With all of the above catalysts available, it is still of importance to develop new chemoselective hydrogenation systems, where the catalysts can be simply prepared. In the following, the authors show that the reaction system NaBH4, CH3CO2H, Pd/C can be used for the hy- drogenation of alkenes without the loss of O-benzyl or N-benzyl groups, so that benzyl ethers, benzyl esters and N-benzyl amides are not converted concurrently to alcohols, acids, and amides, respectively. 2. Experimental 2.1. Chemicals and Instruments Melting points were measured on a Stuart SMP 10 melting point apparatus and are uncorrected. Infrared spectra were measured with a Thermo/Nicolet Nexus 470 FT-IR ESP Spectrometer. 1H and 13C NMR spectra were rec- orded with a Varian 400 NMR (1H at 395.7 MHz, 13C at 100.5 MHz) and a Varian 200 MHz NMR spectrometer 1 13 ( H at 200.0 MHz, C at 50.3 MHz). The chemical shifts are relative to TMS (solvent CDCl3, unless otherwise noted). Mass spectra were measured with a JMS-01-SG-2 spectrometer. CHN-analysis was performed on a LECO TruSpec Micro instrument. Column chromatography was carried out on silica gel (60 A, 230 - 400 mesh, Sigma-Aldrich). 5w% Palladium on carbon (Aldrich, 205680) was used in all experiments. NaBH4 and acetic acid were ac- quired commercially. Benzene, toluene and THF were used without prior purification. Benzyl esters 12, 16, 18, 36, and 40 were prepared from the corresponding acids (benzyl alcohol, PPh3, BrCCl3, CH2Cl2) following a known procedure [12]. Methyl ester 14 was prepared by Wittig olefination from 3-benzyloxy-4-methoxyben- zaldehyde and methoxycarbonylmethylidenetriphenylphosphorane in a minimal amount of CHCl3. Also, N-benzyl amides 27, 29, 31 and 33 were synthesized from the corresponding acids (benzylamine, PPh3, BrCCl3, CH2Cl2) [12]. Substituted dibenzyl ethers 38 and 43 were obtained by Wilkinson-type etherification (ArCH2OH, benzyl chloride, KOH, DMSO) as was 2-benzyloxycinnamaldehyde (24) (2-hydroxycinnamaldehyde, benzyl chloride, KOH, DMSO). 20 and 22 were prepared by Wittig olefination, starting from 2-benzyloxybenzaldehyde and benzoylmethylidenetriphenylphosphorane and from 2-benzyloxycinnamaldehyde (24) and toluoylmethylidene- triphenylphosphorane. Caution: In the presence of dry palladium on carbon, hydrogen enflames upon contact with air. Therefore, it is advisable to purge the reaction flasks with an inert gas before use in the described hydrogenation. Also, where filtrating the reaction mixture directly, especially when using a paper filter, it must be noted that the filter cake upon drying can enflame due to the fact that unreacted sodium borohydride slowly hydrolyses with air moisture, thereby releasing hydrogen. Therefore, after diligent washing with chloroform, the filter and filter cake should be immersed in water. 2.2. General Procedure for the Hydrogenation of Cinnamates.-Methyl 3-[2-Benzyloxyphenyl]Propionate (13) [13] To a solution of methyl o-benzyloxycinnamate (6, 188 mg, 0.70 mmol) in benzene (10 mL) is given Pd/C (70 2 N. A. Soom, T. Thiemann mg, 5 wt%) and acetic acid (AcOH, 100 mg). Thereafter, is added portionwise NaBH4 (128 mg, 3.38 mmol). After 3 h at rt, further AcOH (50 mg) and NaBH4 (60 mg, 1.58 mmol) are added successively, and the resulting mixture is stirred at rt for 12 h. Thereafter, half conc. aq. HCl is added dropwise until there is no futher gas evo- lution. H2O (30 mL) is added and the mixture is extracted with CH2Cl2 (3 × 20 mL). The combined organic phase is dried over anhydrous MgSO4, concentrated in vacuo and the residue is subjected to column chromato- −1 graphy on silica gel (CH2Cl2) to give 13 (175 mg, 93%) as a colorless oil; νmax (neat/cm ) 3064, 3033, 2950, 1736, 1601, 1588, 1493, 1453, 1436, 1381, 1290, 1241, 1193, 1162, 1025, 752; δH (400 MHz, CDCl3) 2.65 (2H, 3 3 t, J = 7.6 Hz), 3.01 (2H, t, J = 7.6 Hz), 3.64 (3H, s, OCH3), 5.09 (2H, s, OCH2), 6.87 - 6.92 (2H, m), 7.16 - 7.46 (8H, m); δC (100.5 MHz, CDCl3) 26.2 (CH2), 34.0 (CH2), 51.5 (OCH3), 69.7 (OCH2), 111.5 (CH), 120.7 (CH), 127.0 (2C, CH), 127.6 (CH), 127.8 (CH), 128.6 (2C, CH), 129.1 (Cquat), 130.1 (CH), 137.2 (Cquat), 156.5 + (Cquat), 173.8 (Cquat, CO); MS (EI, 70 eV) m/z (%) 270 (M , 85). 2.3. General Procedure for the Hydrogenation of Cinnamides.-N-Benzyl 3-Phenylpropionamide (28) [14] To a mixture of N-benzyl cinnamide (335 mg, 1.41 mmol) and Pd/C (70 mg, 5 w%) in toluene (8 mL) was add- ed acetic acid (210 mg) and subsequently NaBH4 (185 mg). After the mixture was stirred for 14 h, it was filtered, and the filter cake was washed with CHCl3 (3 × 15 mL). The combined organic phase was concentrated in vacuo, and the residue was subjected to column chromatography on silica gel (ether/CHCl3/hexane 2:2:1) to give 28 −1 (315 mg, 95%) as a colorless solid, mp. 90˚C - 93˚C; νmax (KBr/cm ) 3292 (s, NH), 3061, 3026, 2924, 1639, 3 3 1543, 1495, 1453, 1227, 1029, 741, 694; δH (400 MHz, CDCl3) 2.51 (2H, t, J = 7.6 Hz), 2.99 (2H, t, J = 7.6 3 Hz), 4.38 (2H, d, J = 5.6 Hz), 5.66 (1H, bs, NH), 7.12 - 7.29 (10H, m); δC (100.5 MHz, CDCl3) 31.7 (CH2), 38.5 (CH2), 43.6 (CH2), 126.3 (CH), 127.5 (CH), 127.7 (2C, CH), 128.4 (2C, CH), 128.6 (2C, CH), 128.7 (2C, CH), 138.1 (Cquat), 140.7 (Cquat), 171.9 (Cquat, CO). 2.4. Reduction of Nitro-Containing Compounds—Variant A: Anthranilic Acid Benzyl Ester (41) [15] To a mixture of benzyl 2-nitrobenzoate (40, 361 mg, 1.4 mmol), Pd/C (100 mg, 5w%) and AcOH (210 mg) in benzene (10 mL) is slowly added NaBH4 (185 mg, 4.87 mmol), and the resulting reaction mixture is stirred at rt for 14 h.
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